It is conventionally known that a catalyst or the like is provided in an exhaust passage in order to purify NOx discharged from an engine.
For example, Japanese Patent No. 3518398 discloses an engine in which an NOx storage and reduction type NOx catalyst is provided in an exhaust passage, and an SCR catalyst is provided downstream of the exhaust passage. The NOx storage and reduction type NOx catalyst stores NOx in an exhaust gas when the exhaust gas is lean where an excess air ratio of the exhaust gas is larger than 1, that is, when an air-fuel ratio of the exhaust gas is larger than a theoretical air-fuel ratio. The NOx storage and reduction type NOx catalyst reduces the stored NOx when the excess air ratio of the exhaust gas is 1 or less. The SCR catalyst purifies NOx in the exhaust gas by reaction with NH3 (ammonia). In this engine, an ammonia compound injection device that injects an ammonia compound serving as a material for NH3 is provided upstream from the SCR catalyst in the exhaust passage. NH3 is generated from the ammonia compound injected by this injection device, and NOx is purified within the SCR catalyst.